PROJECT SUMMARY Trial and use of e-cigarettes among youth who are not users of combustible cigarettes has reached epidemic levels, and e-cigarette product advertisements (ads) have likely played a role. The widespread use of e- cigarettes poses significant harms to the health and well-being of adolescents. Increasing the strength of warning labels on ads for other tobacco products have been found to reduce the appeal of these products among young people, suggesting that they may also be an effective strategy to curb this epidemic among adolescents. One major challenge to the design of stronger and more effective health warnings on e-cigarette ads, however, is the risk that smokers of combustible cigarettes may be deterred from switching when they could experience major health benefits. Warnings therefore must communicate the risks of e-cigarettes to youth and non-smokers while also protecting perceptions of the benefits of switching completely to e-cigarettes among combustible cigarette smokers. This paradox presents a challenge for regulators. The goal of this series of studies is to inform the FDA on issues pertaining to e-cigarette warning regulations given this complex population paradox. Targeted outcomes include preferential visual attention to warnings over ad content, intentions to use both e-cigarettes and combustible cigarettes, and risk beliefs about both tobacco products. Aim 1 will develop and test a set of candidate warning messages to maximize desirable outcomes among two populations within the paradox: nonsmoking youth and adult smokers. Aim 2 will experimentally test warnings on e-cigarette ads to maximize favorable effects on youth as the critical at-risk population. Using our state-of-the-art mobile lab outfitted with computing and eye-tracking technology, we will test the effects of promising warnings from Aim 1 in a randomized experiment with a 4between (candidate warnings) x 4within (ad appeals) design, N=400. We will identify warning labels that increase visual attention to the warnings, decrease attention to ad appeals, increase risk beliefs, and reduce use intentions. Aim 3 will experimentally test for unintended effects of warnings on e-cigarette with adult cigarette smokers who may be discouraged from switching to e-cigarettes when exposed to some types of warnings. Using the same mobile lab, we will conduct a parallel randomized experiment with the same 4between (candidate warnings) x 4within (ad appeals) design, N=400, to test whether the most effective warnings among youth that emerge in Aim 2 have any unintended consequences among adult smokers. We will test whether youth-effective warning labels influence visual attention, comparative risks between combustible and e-cigarettes, and intentions to use both products (switching completely to e-cigarettes, dual use, or continued smoking of combustible cigarettes) among adults.